1. Field of the Invention
The present invention relates to a laser beam oscillating apparatus, which is used in an information processing apparatus such as an optical memory disc system or a laser beam printer, and an optical application measuring instrument.
2. Description of the Prior Art
Recently, because of its easy condensing and superior directivity, laser beams generated by a semiconductor laser device have been used for an information processing apparatus such as an optical memory disc system or a laser beam printer, an optical application measuring instrument and other related apparatus both for industrial and household uses. In addition, because of its small-size and oscillating ability the semiconductor laser devices find many applications in information processing apparatus. The semiconductor laser beams used for such applications are normally red or near infrared having a long wavelength of 680 nm to 1550 nm. Under the recent trend of displaying colored pictures on a laser display such as semiconductor laser devices, a new type of oscillator capable of oscillating green, blue or other color laser beams than red is being developed.
One of the methods of obtaining shortwave laser beams using a small sized device is to covert the optical wavelength. One known method using this technique utilizes the generation of sum frequency wave in which frequencies of plural optical eaves are summed, specifically second harmonics or third harmonics in which two or three of same optical frequencies are summed, respectively. For example, Applied Physics, vol. 56, 12, 1637-1641(1987) reports that second harmonics generate with 1% converting efficiency from the semiconductor laser beams having the wavelength of 840 nm and the output of 40 mW using an optical waveguide formed on a substrate made of LiNbO.sub.3 by means of the proton-exchange method. The report describes that when the semiconductor laser beams are propagated within the optical waveguide having the width of 2.0 .mu.m and the depth of 0.4 .mu.m, a blue harmonic wave having the wavelength of 420 nm formed on the substrate are emitted into the substrate at the slant angle of approximately 16.2.degree. according to the Cerenkov radiation. When the fundamental waves are YAG laser beams having the wavelength of 1060 nm, green laser beams are obtained though, in this case, the radiation angle is slightly different from that of the blue laser beams having the wavelength of 425 nm.
However, the three primary colors of red, green and blue are required to display a color image on a display device. For this purpose, it is possible to use three optical wavelength converters each oscillating laser beams of one of the three primary colors and to superpose the thus oscillated laser beams of three colors by means of a beam splitter, etc. This method, however, needs a large scale apparatus, negating the advantage of using the semiconductor laser device.
Gas laser apparatus such as a He-Cd laser have realized laser beam oscillating devices capable of emitting laser beams of different wavelengths, which are, however, disadvantageously very large in size and short in duration.
In addition, it is dangerous and must be careful to radiate invisible beams such as near infrared beams emitted from the semiconductor laser beam device at the high output like several tens of mW. If such invisible laser beams be emitted together with visible beams in the same direction, they can be directionally controlled simultaneously when the visible beams are controlled, thereby ensuring notable improvement of safety.